Polyamide resins based on homopolymers of 11-aminoundecanoic acid (A11) (it also being possible for these homopolymers to be referred to by the abbreviation PA11) and those based on homopolymers of 12-aminododecanoic acid (A12) or of dodecanelactam (L12) (it also being possible for these homopolymers to be referred to by the abbreviation (PA12) have good properties such as resistance to chemical attacks, flexibility, good dimensional stability due to a low moisture uptake, good resilience, especially at low temperature, and good extrudability.
In the case of the abovementioned applications, investigations are at present being carried out into how to improve further the property of low-temperature resilience and the property of suppleness, especially when plasticizer is added to copolyamides, and to shift the ductile/brittle transition towards a lower temperature.
It has been now surprisingly discovered that this objective can be reached by modifying PA11 with units originating from A12 or L12 and, vice versa, by modifying PA12 with units originating from A11, the monomer compositions of the resulting copolyamides (copolyamides which can be referred to by the abbreviations CoPA 11/12 and CoPA 12/11 respectively) furthermore lying within quite precise ranges, and the said copolyamides exhibiting melting points lying within ranges which are also quite precise. Furthermore, the present invention offers the additional advantage that the CoPA 11/12 copolyamides become much more transparent in relation to the homopolymer resin. As far as known, the copolyamides exhibiting these characteristics have never been described in the literature.
With respect to the literature, copolyamides obtained by polymerization of a mixture of 72 to 82% by weight, preferably 77% by weight, of L12 or of A12 and of 28 to 18% by weight, preferably 23% by weight, of A11, are described in German Patent Application DE-A-2 147 420. However, these copolyamides of CoPA 12/11 type cannot be employed for the applications referred to at present because they are too soft, having too low a melting point (160.degree. C. in the case of CoPA 12/11 of composition 77/23 by weight) and too low a degree of crystallinity. This low degree of crystallinity gives good transparency, but greatly impairs the thermomechanical properties and chemical resistance.
Japanese Patent Application JP-A-04 325 159 relates to 11/12 or 12/11 CoPAs which can be used for the manufacture of blood pouches; the particular weight compositions 65/35 and 82/18 illustrate the 11/12 and 12/11 CoPAs respectively; the melting point is 160.degree. C. in both cases.